diff --git a/scripts/test-free-shards.ts b/scripts/test-free-shards.ts
index 11187876c..1762bbee3 100755
--- a/scripts/test-free-shards.ts
+++ b/scripts/test-free-shards.ts
@@ -349,11 +349,12 @@ export const RESERVED_CPUS = 2;
 export function fullSuiteJobs(): number {
   const raw = process.env.GSTACK_FREE_JOBS;
   if (raw !== undefined && raw !== '') {
-    const value = Number.parseInt(raw, 10);
-    if (!Number.isInteger(value) || value <= 0) {
+    // Strict digits-only: parseInt would silently truncate "2abc" -> 2 and
+    // "3.7" -> 3, defeating the loud-failure contract the error text claims.
+    if (!/^\d+$/.test(raw.trim()) || Number.parseInt(raw, 10) <= 0) {
       throw new Error(`GSTACK_FREE_JOBS must be a positive integer, got: ${raw}`);
     }
-    return value;
+    return Number.parseInt(raw, 10);
   }
   return Math.max(1, Math.min(MAX_FULL_SUITE_JOBS, os.cpus().length - RESERVED_CPUS));
 }
diff --git a/test/test-free-shards-sandbox-knobs.test.ts b/test/test-free-shards-sandbox-knobs.test.ts
index 1de5b81b7..92279d27a 100644
--- a/test/test-free-shards-sandbox-knobs.test.ts
+++ b/test/test-free-shards-sandbox-knobs.test.ts
@@ -55,7 +55,9 @@ describe('test-free-shards: fullSuiteJobs (GSTACK_FREE_JOBS override)', () => {
   });
 
   test('zero, negative, and non-numeric values throw loudly instead of silently defaulting', () => {
-    for (const bad of ['0', '-2', 'abc', 'NaN']) {
+    // '2abc' and '3.7' pin the strict digits-only check: parseInt would
+    // silently truncate them to 2 and 3, defeating the loud-failure contract.
+    for (const bad of ['0', '-2', 'abc', 'NaN', '2abc', '3.7']) {
       expect(() => withJobsEnv(bad, fullSuiteJobs)).toThrow(/positive integer/);
     }
   });